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Science: Meteorites brought water from Mars down to Earth

4 April 1992

By SIMON MITTON

Mars may not always have been dry, claim planetary geologists who have
succeeded in extracting water from meteorites believed to have come from
Mars. ‘A distinct reservoir of water once existed on the surface of Mars,’
says Everett Gibson of the Johnson Space Center, Houston.

Images of Mars taken by spaceprobes clearly show dry riverbeds and canyons.
The features strongly suggest that water once ran freely across the surface.
Today the Martian atmosphere contains only tiny amounts of water, so where
did all the water go? The most likely explanation, say planetary scientists,
is that most of Mars’s ancient oceans now lies beneath the surface of the
planet.

Gibson and his colleagues extracted a water drop from slices of six
meteorites obtained from museum collections. They presented the results
of their analysis of the water at the 23rd Planetary Science Conference
in Houston.

The six meteorites, all examples of a rare type known as SNC meteorites,
are believed to come from Mars because they contain noble gases in precisely
the same ratio as rocks on the Martian surface which were analysed by NASA’s
Viking lander.

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Gibson’s team heated the meteorite samples – no more than few grams
– gently to extract any water trapped in micropores of the rock. They were
careful not to heat the rock too fiercely in case this forced water within
the silicates to break free of its chemical bonds. A few milligrams of water
were released, and was analysed

The ratios of oxygen isotopes in the water showed that it could not
have come from the silicates in the meteorites, because they display a different
isotopic ‘signature’. According to Gibson, the drop of trapped water contained
more of the isotope oxygen-17 than the water in silicates.

Gibson and his colleagues suggest three possible sources for the water
trapped in the meteorites. It could have come from a Martian ocean, from
water vapour in the Martian atmosphere, or from an icy comet that struck
the surface of Mars. Gibson says that the comet theory is ruled out because
the water is present in all six meteorites. An ancient Martian ocean is
the best explanation, he says (Science, vol 255, p 1409).

In another presentation at the Planetary Science Conference, French
space scientists outlined their plans to send robot explorers to Mars. These
would crawl over the surface up to 500 kilometres from a base station, and
one of their tasks would be to set up a seismic experiment to detect any
ice below the surface.